SnTe Layers Research Articles

Molecular beam epitaxial growth of EuTe/SnTe strained superlattices

In this work, we report on the growth of EuTe/SnTe superlattices (SLs) on BaF 2 ( 1 1 1 ) substrates by molecular beam epitaxy. Reflection high energy electron diffraction allowed to find the growth modes at each growth stage, while high resolution X-ray diffraction (HRXRD) was employed to evaluate the heterostructure crystalline quality. In four subsequent stages, we studied the growth of SnTe buffer layers, the growth of EuTe on SnTe, the growth of SnTe on EuTe, and finally undertook the SLs growth. EuTe grows in the Stranski–Krastanov mode on SnTe, with a critical thickness for island formation that increases for lower temperatures and higher Te/Eu flux ratios. On the other hand, SnTe starts growing in the Volmer–Weber mode on EuTe, with evolution to layer-by-layer growth after the SnTe islands merge, which occurs faster for higher temperatures. As confirmed by HRXRD ω / 2 θ scans measured along the (2 2 2) SL Bragg peak, high structural quality EuTe/SnTe SLs were grown with EuTe layers below the critical thickness for island formation and SnTe layers thick enough for layer-by-layer growth to be restored. The X-ray spectra were fitted with those calculated using dynamical diffraction theory in order to find accurately the individual layer thicknesses and strain status.

Investigation of Phase Transition in Stacked Ge-Chalcogenide/SnTe Phase-change Memory Films

ABSTRACTPhase transitions in stacked GeTe/SnTe and Ge2Se3/SnTe thin layers for potential phase-change memory applications have been investigated by X-ray diffraction using a two-dimensional area detector system. The as-deposited underlying GeTe or Ge2Se3 layer is amorphous, whereas the top SnTe layer is crystalline. In the GeTe/SnTe stack, the crystallization of GeTe phase occurs near 170°C, and upon further heating, the GeTe phase disappears, followed by the formation of rocksalt-structured GexSn1−xTe solid solution. In the Ge2Se3/SnTe stack, the phase transition starts with the separation of a SnSe phase due to the migration of Sn ions into the Ge2Se3 layer. SnSe is believed to facilitate the crystallization of Ge2Se3-SnTe solid solution at ∼360°C, which is much lower than the crystallization temperature of Ge2Se3, therefore consuming less power during the phase transition.

Phase-change memory devices with stacked Ge-chalcogenide/Sn-chalcogenide layers

Non-volatile memory devices with two stacked layers of chalcogenide materials comprising the active memory device have been investigated for their potential as phase-change memories. The devices tested consisted of GeTe/SnTe, Ge2Se3/SnTe, and Ge2Se3/SnSe stacks. All devices exhibited resistance switching behavior. The polarity of the applied voltage with respect to the SnTe or SnSe layer was critical to the memory switching properties, most likely due to the voltage induced movement of either Sn or Te into the Ge-chalcogenide layer.

Unusual cruciform pattern interfacial reactions in Sn/Te couples

Unusual cruciform patterns are found in the Sn/Te reaction couples reacted at 250 °C. The cruciform pattern is composed of porous SnTe reaction layers of very uniform thickness along the four sides, and no reaction phase was observed at the corners of the rectangular Te substrate. The reaction SnTe layer cracks at the corners due to its inherent grown stress. The unusual cruciform pattern results from the fast growth rate, and the porous and brittle natures of the SnTe reaction phase.

Characterization of SnTe films grown by molecular beam epitaxy

A series of SnTe layers with thicknesses varying from 0.42 to 9.1 µm were grown by molecular beam epitaxy on (111) BaF2 substrates. The SnTe lattice parameter was found to be 6.331 Å as determined from x-ray diffraction spectra measured in the triple-axis configuration. The FWHM of the (222) SnTe x-ray rocking curves indicated a good crystalline quality and an unusual dependence on layer thickness. Atomic force microscopy (AFM) of the SnTe surface revealed spirals with monolayer steps formed around threading dislocations, similar to the PbTe on BaF2 epitaxy. The dislocation density was estimated from the AFM picture to be 9x10(8) cm-2. Small black pits corresponding to holes that were left during growth were also observed on the AFM images. Sn diffusion can be a possible reason for these pits and the relatively high dislocation density. Electrical measurements showed that the SnTe epilayers present a typical p-type carrier concentration around 10(20) cm- 3 almost temperature independent and a Hall mobility which decreases from 10(4) to 10³ cm²/V.s as the temperature increases from 10 to 350K.

Growth mechanism and thermoelectric properties of PbTe/SnTe/PbTe heterostructures

An electron microscopy study of the mechanism of SnTe and PbTe layer growth in PbTe/SnTe/PbTe heterostructures prepared by thermal evaporation in vacuum onto a KCl substrate was performed. It is established that PbTe and SnTe grow on one another in a layer-by-layer fashion with the introduction of misfit dislocations on the interface at a critical thickness dc≈2 nm. The experimentally determined dependence of the elastic stress in a growing layer (either PbTe or SnTe) on the layer thickness and the critical thickness are in good agreement with those calculated theoretically. The dependences of the thermoelectric properties of PbTe/SnTe/PbTe heterostructures on the SnTe layer thickness (dSnTe=5–100 nm) at fixed thicknesses of the PbTe layers were studied at room temperature. It was found that in the thickness range of dSnTe≈(10–15) nm, an inversion of the dominant carrier sign from n to p takes place. The d-dependences of the thermoelectric properties were interpreted within the framework of a three-layer model, treating a PbTe/SnTe/PbTe heterostructure as three conductors connected in a parallel fashion, each characterized by its specific electrophysical parameters.

Thickness Dependences of the Thermoelectric Properties in (001)KCl/PbTe/SnTe/PbTe Heterostructures

ABSTRACTThe dependences of the thermoelectric properties of (001)KCl/PbTe/SnTe/PbTe three-layer structures on the SnTe layer thickness (dSnTe = 0.5–6.0 nm) at a fixed thickness of PbTe layers were studied. It was established that the thickness dependences of the Seebeck coefficient, the Hall coefficient, electrical conductivity, charge carrier mobility, and the thermoelectric power factor are distinctly non-monotonic. Two possible reasons for this non-monotonic behavior of the thickness dependences of the thermoelectric properties are considered: the size quantization of the energy spectrum in a SnTe quantum well and / or the formation of edge misfit dislocations at the interfaces after reaching the critical thickness, which corresponds to the transition from a pseudomorphic growth to the introduction of misfit dislocations at the interfaces. It is suggested that the observed effect has a general character and should be taken into account when optimizing thermoelectric properties of superlattices.

Structural and Electrical Properties of Low Concentration SnTe Layers and PbTe/SnTe Heterostructures Grown by MBE

We analyse properties of thin SnTe layers and PbTe/SnTe heterostructures grown by MBE on BaF2(111) substrates. Reflection high energy electron diffraction patterns registered during MBE growth of the samples aś well as post-growth X-ray diffraction measurements evidence a high structural perfection of 0.6 μm thick SnTe layers and (50A PbTe)/(50 A SnTe) superlattices. The full width at half maximum values of (222) X-ray rocking curves measured for these thin SnTe layers crystallized in the optimal MBE growth conditions are about 300 arcsec; the carrler concentrations can be tuned from 5 x 10 19 cm-3 to 1021 cm-3 depending on the MBE process parameters. PACS numbers: 81.15.Gh, 61.14.Hg

X-ray Characterization of PbTe/SnTe Superlattices

AbstractPbTe/SnTe superlattices have been proposed many years ago for use as base material for infrared detectors. However, many difficulties have prevented its use, mainly the ones related to obtaining low concentration SnTe. Recently we have shown that SnTe layers with relatively low hole concentration can be grown by molecular beam epitaxy at low temperature using stoichiometric charges. In this work we investigate the structural properties of PbTe/SnTe superlattices grown by molecular beam epitaxy on (111) BaF2 substrates. Sample characterization has been done by high resolution x-ray diffraction. Information on strain was obtained from reciprocal space maps of asymmetric Bragg reflections and used as input parameters for dynamical simulation of the diffraction spectra.

Stoichiometric defects and volume diffusion in epitaxial PbTe‐SnTe multilayers

AbstractThe effect of annealing on multilayer structures based on epitaxial layers of PbTe‐SnTe was investigated by X‐ray diffraction profile changes. Decrease of satellite reflections intensity and conversion of pattern asymmetry were found. Profiles of interplanar distance and electron density deviations were determined by solving both the direct problem (numerical computation of Takagi‐Taupin equations) and the inverse problem (optimisation task) according to the average lattise model. Computation showed the presence of interlayer area of PbSnTe composition containing up to 8% of stoichiometric vacancy type defects between PbTe and SnTe layers. Two diffusion mechanisms are proposed: “fast” diffusion connected with occupation of stoichiometry defects and “slow” substitutional diffusion.

Strain in PbTe/SnTe heterojunctions grown on (001) KCI

PbTe/SnTe heterojunctions have been epitaxially grown on (001) KCI. The effects of strain at the PbTe/SnTe interface have been investigated with the help of electron microscopy. A square grid of edge misfit dislocations with an irregular period has been observed at the interface. The misfit dislocations are generated only in SnTe (or PbTe) layers thicker than some critical thickness, h c ≊ 10 nm . The dislocation period decreases with an increase in the layer thickness. The experimental results have been compared with the theory of elastic strain in epitaxial overgrowths. The theory predicts considerably smaller strain and critical thickness for the introduction of dislocations. Possible reasons for the discrepancy are discussed.

Superconductivity of Non-Strained PbTe-PbS and Strained PbTe-SnTe Superlattices

A comparative study of structural and superconductive properties of semiconductor epitaxial superlattices PbTe-SnTe and PbTe—PbS grown on (001) KCl has been carried out. It has been found that the superconductivity of the PbTe—SnTe superlattices is caused by the stretching strain in the SnTe layers and it may be connected with the relative positions of L+8 and terms of PbTe and SnTe, respectively in the heterojunction. In contrast to the PbTe—SnTe superlattices, the superconductivity of the PbTe—PbS superlattices is found to be associated with regular misfit dislocation grids which are generated at the interfaces.

The galvanomagnetic properties of short-period [formula omitted] superlattices

(001) oriented SnTe PbTe superlattices (SLs) with the period in the range of 9–60 nm are grown. The temperature dependence of the conductivity, the Hall coefficient and the magnetoresistance anisotropy are investigated in the SLs in the temperature range of 1.5–300 K and magnetic fields up to 1.5 T. The SLs are of n-type with the apparent electron concentration in the range of 10 19–10 20 cm −3. The type II band ordering is concluded for the SLs. Below 6 K the SLs exhibit superconductivity. The superconductivity is suggested to have a local character and to be associated with the strained pseudomorphic SnTe layers.

PbTeSnTe superlattices

The potential of the PbTeSnTe superlattice as an infrared material is examined. The expected band gap as a function of repeat distance and layer thickness ratio has been calculated for a small period superlattice. Useful band gaps may be obtainable with physically realizable layer thicknesses. Superlattices with 60 Å PbTe and 60 Å SnTe layers were produced by vacuum deposition onto BaF 2 substrates. While undoped superlattices are strongly p-type due to Sn vacancies, doping the SnTe layers with 1 at% Bi produces n-type superlattices. The optical gap of these superlattices is found to be 0.20 eV, much larger than the calculated value of about 12 meV. The optical gap may be enlarged by a Burstein shift.

Heterolayer lead-salt diode lasers

Hot-wall epitaxy (HWE) has been used to grow heterostructure lead-salt materials from which low-threshold tunable diode lasers have been made. A new HWE structure consisting of a Pb(Se, Te) layer sandwiched between two lattice-matched (Pb, Sn)Te layers has resulted in lasers of good electrical and material quality, and threshold current densities as low as 200 A∙cm−2 (at 40 K). This occurred even though this structure is expected to be nonconfining to both light and electrical carriers. This result is due to the very rapid interdiffusion of dopant atoms between the epilayers during the growth process. Dopant interdiffusion has been investigated using an etch-back technique combined with hot-point probe measurements to observe changes in the doping profiles of the structures. Very large values for the diffusion constants of dopants have been deduced from these measurements: 2.3 × 10−15 and 1.1 × 10−15 cm2∙s−1 for Bi and Tl, respectively.

Influence of angular misorientation of thin Sn/SnTe bilayers of the fine structure of electron diffraction patterns

The peculiar electron diffraction patterns of Sn/SnTe specimens grown by evaporation on a NaCl substrate were studied. The streaks formed by doubly diffracted beams could be explained by simple rotations of the lattices. It is shown that the pattern of the streaks allows one to determine more reliably which of the layers is more misoriented than the arcing of the basic reflections. In most of the investigated specimens the rotational misorientation of the SnTe layer is found to be larger than that of the deposited Sn islands. The possible origins of moiré fringe irregularities observed in the coalescence stage of Sn islands are discussed.